1. Field of the Invention
The present invention generally concerns image generators where an energy beam is selectively turned on and off at clocked intervals while it is scanned across a markable media in order that it may selectively mark the media.
The present invention more specifically concerns image generators where, because the energy beam is scanned across the media at a non-uniform velocity, the clocked intervals are nonuniformly separated, and the clocking waveform is of a variable frequency.
The present invention still more specifically concerns (i) the phase synchronization of a fixed-phase clock to an asynchronous, sensed, occurrence of a start-of-scan line condition, and (ii) the digital generation of a variable-frequency clock waveform from a fixed-frequency clock.
2. Description of the Prior Art
In a beam-scanning imaging device, such as a laser printer, an energy beam, normally a laser light beam such as is derived from a helium-neon laser or a laser diode, is swept across a photoconductive target such as a photoconductive drum or photoconductive paper, or a photosensitive target, such as photographic film or plates. Diverse ways of scanning the light beam exist. These include motor-driven polygonal mirrors, spinning holographic disks, a linear galvanometer and mirror, or a self-resonating galvanometer scanner. A survey of these methods is contained in the article "Laser Scanning and Recording: Developments and Trends" occurring in LASER FOCUS/ELECTRO-OPTICS for February 1895 at pages 88-96.
The motion of the marker, or laser, energy beam across the markable media is typically not linear. In a self-resonating galvanometer scanner imaging system, in particular, the motion of the scanning mirror, and the swept energy beam, is periodic and sinusoidal. The swept energy, or laser, beam typically traverses the media at a maximum scan velocity at or near the center of each imaged line, and the center line of the imaged area. Conversely, the minimum scan velocity is at or near the ends of each imaged line, and the side borders of the imaged area.
Because of the variable velocity of the scanning energy beam across the imaged media, the imaged pixels must placed upon the media at differing time intervals, one to the next. This requires a variable-frequency clock.
The placement of pixels on the media is responsive to timing signals, or a timing chain, that is, consequent to the non-regular times and non-uniform durations at which pixels are placed, of a varying frequency. A common prior art means of deriving such a variable-frequency pixel placement timing chain was a voltage controlled oscillator (VCO). AVCO has the advantages that it may be both (i) started and stopped, producing thereby a controllable beginning and end to the oscillations, or timing chain, that is generated, and (ii) adjusted, or cycled, in frequency during oscillation by a variation in its input d.c. voltage.
Although a VCO may be straightforwardly adapted to produce a variable-frequency pixel placement timing chain, it suffers form problems of stability and repeatability. Nonetheless that a pixel placement timing chain should of a variable frequency, it is desired to be both (i) highly accurate and (ii) highly repeatable during a single scan line, and from one scan line to the next. If the pixel placement timing chain is not accurate the individual pixels will not be placed in the precisely correct locations along the scan line, and/or will be of irregular and non-uniform size. if the pixel placement timing chain is not repeatable then the pixels upon one line will be at different locations than the corresponding pixels that are upon preceding, or succeeding, lines, causing a visually detectable flaw in the image. These problems are generally most severe in the region of the image that is the last to be produced, which is generally at its right side. These problems are generally more severe if the image is large, slowly generated, and/or contains many pixels per inch--permitting a greater accumulation of error before the commencement of a new line scan imaging cycle.
In a related area, it is known in a swept-beam imager, such as in an imager deflecting a laser beam by use of a self-resonant galvanometer scanner, to use a photo diode to detect the start of each scan line. One use of such a photodiode is taught in U.S. Pat. No. 4,686,363 to Schoon for a SELF-RISONANT SCANNER BIASING SYSTEM. The start-of-scan photo diode produces an synchronous signal to which the variable-frequency pixel placement timing chain must normally be synchronized in order to accurately produce an image.